This invention relates generally to organic light emitting device (OLED) displays that have light emitting layers that are semiconductive polymers or small molecules.
OLED displays use layers of light emitting materials. Unlike liquid crystal devices, the OLED displays actually emit light, making them advantageous for many applications.
OLED displays may use either at least one semiconductive conjugated polymer or a small molecule sandwiched between a pair of contact layers. The contact layers produce an electric field that injects charge carriers into the OLED layer. When the charge carriers combine in the OLED layer, the charge carriers decay and emit radiation in the visible range.
It is believed that some OLED compounds containing vinyl groups tend to degrade over time and use due to oxidation of the vinyl groups, particularly in the presence of free electrons. Since driving the display with a current provides the free electrons in abundance, the lifetime of the display is a function of applied current between an anode and cathode. Newer compounds based on fluorine have similar degradation mechanisms that may be related to chemical purity, although the exact mechanism is not yet well known in the industry.
In general, OLED displays have a lifetime limit related to the total integrated charge passed through the display. Thus, the luminance of OLED displays generally decreases with use. In order to achieve a desired luminance for a given pixel at a given time in the course of the display's lifetime, the OLED luminance versus current characteristics for a particular manufacturing process are well characterized as a function of aging. For a given total integrated charge, the device current needed to achieve a specific luminance is therefore known.
A matrix display comprises many individually addressable pixels. For a particular type of emissive display comprising OLEDs, each pixel comprises OLED devices addressed by rows and columns. Colors are typically implemented in an OLED display by incorporating in each pixel, individually addressable “sub-pixels” of red, green, and blue.
The primary colors in a linear physical intensity color space, such as the Commission Internationale de l'Eclairage (CIE) xy (1931), form a color gamut which, in some cases, inscribe the vertices of a triangle. Any coordinate inscribed by the gamut identifies a color that can be represented by the scaling of the intensity of each primary color. Embodiments of the present invention are applicable to color spaces that include three or more colors.
The human eye is sensitive to color differences. The perceptible difference between two colors can be described within the well known CIE “color space” which is represented as a plane diagram in units of )-C*, where one )-C* is the just noticeable difference (the color difference in units of x-y which is just noticeable varies depending on the x-y coordinates of the color).
In the course of aging, the luminance for a given drive current decreases non-linearly. Moreover, the nature of the change of luminance over lifetime is more complex than even the non-linear relationship between luminance and drive current. In addition, individual colors change differently in the course of display lifetime. Thus, simply changing the drive current to achieve a desired characteristic luminance may be insufficient. For example, color variations between the many pixels may become perceptible, creating the distracting artifact known as fixed pattern noise. Thus, if, initially or at any time thereafter, sub-pixels of a given color are not exactly the same, fixed pattern noise may arise.
In addition, in the course of aging, the individual sub-pixels may change color differently as a result of aging. If the OLED colors change during aging and all the sub-pixels do not age in substantially the same way, a color difference may become perceptible. This may be especially problematic in an application where static images are displayed including displays utilized for signs.
Thus, there is a need for a better way to compensate for static and dynamic changes in color from sub-pixel to sub-pixel in OLED displays.